The present invention relates generally to terminations for transmission cables and more particularly to a strain relied, pull-strength termination with controlled cable impedance usable in a measurement test probe.
A commonly used design for measurement test probes is an electrically conductive elongate body made of nickel plated brass having a substrate disposed therein. The substrate contains either passive or active circuitry to minimize probe loading and to terminate the probe in the characteristic impedance of the measuring device. The elongate electrically conductive body has a probing tip at one end that is secured in the body by and insulating plug. The probing tip extends through the insulating plug and is exposed within the conductive body. The substrate is electrically connected to the probing tip by electrically conductive elastomeric material or other such conventional connecting methods. Surrounding a substantial portion of the conductive body is an insulating material, such as injected molded plastic parts. Various methods are used for attaching the plastic parts to the tubular body, such as press fitting, gluing, or injection molding of the plastic directly onto the body. The plastic parts are generally formed with an outwardly extending flange that serves as a finger stop and guard. The plastic parts also serve as an outer housing for securing a transmission cable to the probe.
The transmission cable has a central conductor encased in an dielectric material and surrounded by an outer conductive shielding material. The outer conductive shielding material is covered by a insulating material. Generally, the outer shielding material is of finely braided wires. The central conductor is electrically connected to the substrate and the outer shielding material is electrically connected to the electrically conductive body. Many apparatus and methods are employed for connecting the transmission cable to the measurement test probe.
One such apparatus and method is described in U.S. Pat. No. 5,061,892, titled "Electrical Test Probe Having Integral Strain Relief and Ground Connection", and assigned to the assignee of the present invention. A strain relief adapter is provided that has a tubular shaped member and a flat surface portion that transitions from the tubular member. The flat surface portion is secured to the substrate of the measurement probe. The outer insulating material of the transmission cable is removed and the outer conductive shielding material is folded back over the outer insulating material and positioned in the tubular member of the strain relief. The substrate and strain relief adapter are inserted into the electrically conductive elongate body and the conductive body is crimped at the location of the tubular member of the strain relief adapter using an appropriate crimping tool to capture and secure the transmission cable within the electrically conductive elongate body.
U.S. Pat. No. 3,828,298, titled "Electrical Terminal for a Braided Shied on a Coaxial Cable", describes and electrical terminal for grounding the braided shield of a coaxial cable. The terminal includes a generally U-shaped braided shield ferrule-forming portion and an integrally formed wire barrel. The wire barrel includes a base portion and a pair of upstanding sidewalls extend from opposite sides of the base portion. The U-shaped braided shield ferrule-forming portion includes a base portion and upstanding sidewalls on either side of the base portion. A pair of lances are stamped out of each sidewalls and are integral with the base portion. A coaxial cable is prepared for the electrical terminal by removing a portion of the outer insulating jacket and exposing the braided shield. A portion of the exposed braided shield is removed to expose the insulating material surrounding the center conductor. A portion of the insulating material is removed to expose the center conductor. The prepared coaxial cable is aligned over the terminal with the exposed braided shield directly over the ferrule-forming portion. The coaxial cable is forced down onto the terminal with the lances piercing the braided shield. The sidewalls of the wire barrel and the ferrule-forming portion are respectively crimped around the insulating jacket and the braided shield.
A major drawback to these and other similar types of design is that the coaxial cable or transmission cable is crimped, in part, to provide the strain relief and pull-strength on the cable. Such crimping causes changes in the characteristic impedance of the coaxial cable or transmission cable at the crimping location. The impedance changes in the coaxial cable adversely affects the overall bandwidth characteristics of the measurement test probe. With the bandwidth requirements of measurement test probes exceeding 3 GHZ., a new design is needed for providing strain relief and pull-strength for transmission cables used with measurement test probes that does not affect the characteristic impedance of the transmission cable.